1. Field of the Invention
This invention relates to electrolysis cells used for the production of light metals, such as magnesium, lithium, sodium and aluminum, by electrolysis of a molten electrolyte having a greater density than the metal produced. More particularly, the invention relates to electrolysis cells of this kind that have means for separating newly formed molten metal from reactive gases produced during the electrolysis process.
2. Description of the Related Art
The production of magnesium metal is typical of the procedures to which the present invention relates. Magnesium metal is generally produced from magnesium chloride by electrolysis in a suitable electrolytic cell held at a temperature high enough to keep both the electrolyte and the metal product molten during the process. The electrolysis creates droplets of magnesium metal and chlorine gas. Since magnesium is a very light metal, it floats to the surface of the electrolyte, as do the bubbles of the chlorine gas. It is therefore necessary to keep the floating pool of metal separate from the chlorine gas collecting in the atmosphere above the electrolyte, or these elements will merely recombine, thereby reducing the current efficiency of the cell.
A typical modern cell design is described in U.S. Pat. No. 5,935,394 to Sivilotti et al. which issued on Aug. 10, 1999 and was assigned to the same assignee as the present application. This cell utilizes a number of multi-polar cell assemblies per cell that are arranged in a line along one long side wall of the cell. As best shown in FIG. 2 of the patent, a longitudinal refractory curtain wall is provided adjacent to the cell assemblies to separate a compartment within the cell for the electrode assemblies from a metal collection compartment. Electrolyte containing metal droplets overflows the top end of the multi-polar electrodes of each assembly (driven by the buoyancy of entrained gas) and flows through an upper aperture in the refractory wall positioned below the surface of the electrolyte. Gas entrained in the electrolyte escapes into the atmosphere above the electrode assemblies before the metal-containing overflow progresses through the aperture in the curtain wall. After passing through that aperture, the electrolyte encounters a quiescent zone where the metal droplets can rise to the surface, coalesce and collect as a pool. A further aperture at the bottom of the curtain wall allows metal-depleted electrolyte to recirculate to the electrode compartment. In this way, the chlorine gas is kept separate from the floating pool of molten metal.
There are, however, a number of disadvantages with this type of cell. Firstly, the electrolyte overflows the interpolar electrodes of each electrode assembly at all points around the upper end of the assembly. The overflowing electrolyte is collected in a trough surrounding the assembly and brought around to a position adjacent to the aperture in the curtain wall so that it can proceed through the aperture into the metal collection chamber. However, electrolyte that overflows the electrodes at points remote from the aperture spend a considerable amount of time in the electrode compartment where the metal droplets may contact the chlorine gas and may react, thus reducing the current efficiency of the cell.
Secondly, the cell has to be shut down if one of the electrode assemblies has to be removed for maintenance or repair because the headspace within the electrode compartment is common to all electrode assemblies, and chlorine gas produced by functioning assemblies would escape from an aperture in the cell opened for the removal of another cell assembly.
Thirdly, the use of an elongated curtain wall extending the full length of the long dimension of the cell reduces the operational life of the cell and limits the maximum cell dimensions. Such walls are exposed to corrosive chemicals on both sides, so that failure is fairly frequent. When such an integral component fails, it is necessary to shut down the cell completely and to remove the cell contents so that the wall can be rebuilt. This is obviously time consuming, difficult and expensive. Moreover, an unduly long wall would be structurally weak and prone to mechanical failure.
As well as exhibiting the problems referred to above, the cell of the Sivilotti et al. patent also has the problem that the electrical bus bar for the cathode connection exits the cell directly through a side wall at a position below the surface of the electrolyte. While this protects the metal bus from attack by chlorine gas, it makes any removal of electrode assemblies more difficult since the cathode connections are not easily accessible.
Accordingly, there is a need for an improved design of electrolysis cells of this kind to alleviate some or all of the problems of this kind.
An object of the present invention is to make electrolysis cells used for the production of light metals, such as magnesium, easier to construct, maintain and/or to repair.
Another object of the present invention is to increase the operational life of electrolysis cells used for the production of light metals, such as magnesium.
A further object of the present invention, at least in preferred forms, is to reduce the contact time between metal and chlorine in electrolysis cells of the kind discussed.
A further object of the present invention, at least in preferred forms, is to make individual electrode assemblies operationally independent of each other within an electrolysis cell.
A still further object of the invention, at least in preferred forms, is to make repair or maintenance of electrolysis cells possible without a complete cell shut-down.
According to one aspect of the invention, there is provided an electrolysis cell for recovery of a metal by electrolysis from a molten electrolyte containing a metal compound, wherein the molten metal has a density lower than the molten electrolyte and the compound produces a gas during electrolysis that reacts on contact with the molten metal, the cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one interpolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs, and connections for conveying electrical current to and from the electrode assemblies; wherein each electrode assembly is provided with a hood enclosing an upper portion of the electrode assembly including the cathode of the assembly, such that the hood in operation provides a gas collection chamber such that the gas generated by each electrode assembly is isolated from other electrode assemblies and from metal collecting in the housing outside each hood.
According to another aspect of the invention, there is provided a method of recovering a metal by electrolysis from a molten electrolyte containing a metal compound, the molten metal having a density lower than the molten electrolyte and the compound producing a gas during electrolysis that reacts on contact with the molten metal, in which electrolysis is conducted in a cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one interpolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs, and connections for conveying electrical current to and from the electrode assemblies; wherein the gas from each electrode assembly is collected in a hood enclosing an upper portion of the electrode assembly including the cathode of the assembly and providing a gas collection chamber, such that the gas generated by each electrode assembly is isolated from other electrode assemblies and from metal collecting in the housing outside each hood.
According to another aspect of the invention, there is provided an integral electrolysis unit comprising an electrode assembly having an anode, a cathode and at least one interpolar electrode, and a hood encircling an upper end of the to electrode assembly, the hood including a lower end sealed in a gas-fight manner against a periphery of the cathode, except at at least one open aperture at a point on the periphery of the cathode.
According to yet another aspect of the invention, there is provided an electrolysis cell for recovery of a metal by electrolysis from a molten electrolyte containing a metal compound, wherein the molten metal has a density lower than the molten electrolyte and the compound produces a gas during electrolysis that reacts on contact with the molten metal, the cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one bipolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs and the cathode forms an electrically and mechanically continuous surface surrounding the outermost at least one bipolar electrode, and connections for conveying electrical current to and from the electrode assemblies; wherein each electrode assembly is provided with a hood enclosing an upper portion of the assembly such that (a) the hood in operation provides a gas collection chamber such that the gas generated by the electrode assembly is isolated from the remaining electrode assemblies and (b) the hood and outer surface of the cathode are in a spaced relationship so that in operation, electrolyte flow containing the metal formed on the electrodes can flow over the top of the cathode and under the edge of the hood substantially adjacent the cathode over which it flowed.
In a preferred form of the cell, a current bus for the cathode of each assembly attaches to the cathode below a lower end of the associated hood and extends within the cell outside the hood to the cell roof, where it exits the cell through an aperture in the roof.